Process for producing optical transmission fiber

ABSTRACT

A process for producing an optical transmission fiber is provided which comprises feeding highly pure halides, hydrides or organic compounds of Si and B by way of carrier gas on the outer surface of a fused silica rod or a fused silica pipe, or inner surface of a fused silica pipe, oxidizing them and depositing the products to form a pure fused silica layer or a doped fused silica layer containing B 2  O 3 , melting the pipe and the deposited layer followed by a spinning. The SiO 2  layer can alternatively contain fluorine instead of B 2  O 3 . A further SiO 2  layer can be deposited thereon to improve the spinning processability and lower the index of refraction of the B 2  O 3  containing layer.

This application is a divisional application of Ser. No. 648,998, filedJan. 14, 1976, now U.S. Pat. No. 4,082,420, in turn a continuationapplication of Ser. No. 419,011, filed Nov. 26, 1973, now abandoned.Ser. No. 930,841 is a divisional application of Ser. No. 648,998 nowU.S. Pat. No. 4,082,420, Apr. 4, 1978.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for producing an opticaltransmission fiber.

2. Description of the Prior Art

Many of the optical transmission fibers in the prior art are made ofoptical glass and show considerable light aborption losses since suchtransmission fibers are more liable to contain impurities as comparedwith those made of a fused silica and are restricted with respect to thepurities of the raw materials and the melting process therefor. Anotherexample of a known optical transmission fiber is a fused silica cladtype fiber. This clad type fiber is produced by depositing a fusedsilica layer doped with metal oxides on the inner surface of a fusedsilica pipe to increase the index of refraction above that of a fusedsilica, sintering the same in an oxygen atmosphere, heating and meltingfor spinning to eliminate the cavity of the fused silica pipe. The fiberis thereafter annealed in an oxygen atmosphere to completely oxidize themetal component.

This heat-treatment weakens the strength of the fiber.

SUMMARY OF THE INVENTION

This invention provides an optical transmission fiber comprising atleast one lower index of refraction portion comprising a doped fusedsilica containing B₂ O₃ or fluorine and at least one higher index ofrefraction portion comprising fused silica.

This invention overcomes the foregoing defects in the prior art and,provides a process for producing an optical transmission fiber in whicha fused silica layer containing B₂ O₃ or F therein is deposited on thesurface of a pure fused silica to decrease the index of refraction fromthat of the pure fused silica.

This invention also provides a process of depositing a further SiO₂layer on the outer surface of the doped fused silica layer therebyobviating the defects caused by the lowering of the melting point in theSiO₂ -- B₂ O₃ system below that of the fused silica and further loweringthe index of refraction in the SiO₂ -- B₂ O₃ portion due to the tensilestress exerted thereon after the formation of the fiber.

In addition this invention provides a process for producing an opticaltransmission fiber in which the heating is effected within temperatureswhich result in less evaporation of B₂ O₃ and enough movement of gasbubbles which are formed at lower temperature that the bubbles can beeliminated under vacuum or by the application of supersonic waves toincrease the content of B₂ O₃ in the SiO₂ -- B₂ O₃ system.

Further this invention provides a process for producing a fiber which isformed by depositing a SiO₂ layer or a water-repellent glass on theouter periphery of the doped fused silica layer containing B₂ O₃ or F soas to inhibit water permeation which may cause destruction in thenetwork structure of the glass.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

This invention is to described in detail referring to the accompanyingdrawings for the illustration of preferred embodiments of this inventionwherein:

FIG. 1(a) shows cross sections of a fused silica rod and a pipe beforespinning;

FIG. 1(b) shows cross sectional structures of preferred embodiments ofthis invention and the distribution of the index of refractioncorresponding thereto in which A represents a clad type fiber, B₁ and B₂are O-shaped optical wave guides and C is a fiber having a parabolicindex of refraction distribution;

FIG. 2 is a schematic view for illustrating, as an example the processfor producing a fused silica rod or pipe to be spun into the fibersshown in FIG. 1(b );

FIG. 3 shows a suitable apparatus for feeding BBr₃ and SiCl₄ by way ofan oxygen carrier to a oxy-hydrogen burner shown in FIG. 2; and

FIG. 4 shows cross sections of other embodiments of this inventioncomprising a further SiO₂ layer or a water-repellent glass layerdeposited on the periphery of the fused silica rod or pipe shown in FIG.1.

DETAILED DESCRIPTION OF THE INVENTION

In FIGS. 1(a) and (b), FIG. 1(b ) represents the cross sectionalstructures of fibers comprising preferred embodiments of this inventionand the index of refraction distributions corresponding thereto and FIG.1(a ) shows the cross sections of a fused silica rod and a pipe beforespinning.

In FIG. 1, are shown a clad type fiber A, optical O guides B₁ and B₂ anda fiber C having a parabolic index of refraction distribution. Highlypure fused silica 1 is surrounded with a layer 2 of a doped fused silicacontaining B₂ O₃. 3 indicates a cavity portion (filled with air hereinin the case of, B₁ or with a doped fused silica containing B₂ O₃ in thecase of B₂. Since the index of refraction in the portion 2 is lower thanthat of the portion 1, optical energy proceeds selectively concentratedin the portion 1. Distribution chart C indicates that the index ofrefraction decreases in the parts of the portion 2 nearer the surfacesince these surface parts contain more B₂ O₃. This is also applicable toa fiber which contains F in the SiO₂ layer with only the differencebeing in the dopants.

FIG. 2 is a schematic view for the illustration of an example of theproduction of the fused silica rod or pipe which is spun into the fibershown in FIG. 1(b). Generally, it is possible to oxidize hydrides,halides or organic compounds of boron and silicon into SiO₂ containingB₂ O₃ together with respective B₂ O₃ and SiO₂, and the SiO₂ -- B₂ O₃component can be deposited on the outer surface of the rod or pipe whichis previously cleaned and smoothened by applying treatments such asmechanical polishing, laser finishing, sapphire polishing, fluoric acidwashing or fire polishing. FIG. 2 shows a suitable apparatus forillustrating such a method in which a fused silica rod or pipe 1 isarranged so that it can be moved reciprocally in the longitudinaldirection of the rod or pipe and can be rotated around the axis of therod or pipe. BBr₃ and SiCl₄ carried in an oxygen stream are fed to anoxy-hydrogen burner 2 with the following reaction as shown belowoccurring at the exit:

    SiCl.sub.4 + 2H.sub.2 + O.sub.2 = SiO.sub.2 + 4HCl

    4BBr.sub.3 + 6H.sub.2 + 30.sub.2 = 2B.sub.2 O.sub.3 + 12 HBr

Simultaneously, the reaction products B₂ O₃ and SiO₂ at a hightemperature deposit in a powder or glass like state on the rod or pipe.

FIG. 3 shows a suitable apparatus for carrying the BBr₃ and SiCl₄ to theoxy-hydrogen burner shown in FIG. 2 in oxygen gas. In this figure,gaseous oxygen is purified in a purifier 1, passed through a flow meter2 and bubbled, through liquid 6 of BBr₃ or SiCl₄ using three-way cocks 3and 4 in an evaporator 5 provided in a thermo-statically controlled bath7. BBr₃ or SiCl₄ vapor is thus carried in the oxygen gas and the gasmixture is fed to a burner.

Although the descriptions have been made for an oxygen carrier gas, itis of course possible to use other carrier gases such as inert gases,hydrogen, etc. as well.

The heat source is not necessarily limited to an oxy-hydrogen flame butan electrical furnace, a high frequency plasma furnace or other furnacescan also be employed.

The melting and spinning of the rod D or pipe E thus produced (the innercavity of which is either left as it is or is filled with a SiO₂ -- B₂O₃ component after cleaning the cavity) shown in FIG. 1(a) results infibers A, B₁, B₂ and C shown in FIG. 1(b). The rod D is spun into fibersA and C, and the pipe E into fibers A and C when the cavity iseliminated and into the fiber B, i.e., B₁ when the cavity is left and B₂when the cavity is filled.

A fiber shown in FIG. 1(a) and having a portion 3 consisting of SiO₂ --B₂ O₃ can be produced by depositing a SiO₂ layer on a cleaned surface ofSiO₂ -- B₂ O₃ and depositing a further layer of SiO₂ -- B₂ O₃ thereon.

It is, of course, possible to form a fiber having a parabolic refractiveindex distribution by deleting the portion 1 in the rod or pipe shown inFIG. 1(a).

The mthod of depositing an SiO₂ glass layer containing F is to bedescribed. Provisions are made for both of an axial reciprocatingmovement and a rotating movement of a rod or pipe 1 of a pure fusedsilica having a cleaned surface in just the same way as previouslydescribed with reference to FIG. 2. SiF₄ gas is fed around the outersurface of the rod or pipe and reacted in accordance with the followingreaction scheme to form SiO₂ whereby F is incorporated into the SiO₂ :

    siF.sub.4 +2H.sub.2 O + O.sub.2 = SiO.sub.2 + 4HF

generally, SiO₂ can be obtained by oxidizing SiF₄, and a minor amount ofF is incorporated then into this SiO₂. SiF₄ can be synthesized, forexample, by the thermal decomposition of well-known highly purecompounds, BaSiF₆, K₂ SiF₆, H₂ SiF₆ or the like, or the reaction betweenSiO₂ and HSO₃ F and the reaction between SiCl₄ and F₂.

Other compounds than SiF₄ can be employed such as halides, hydrides andorganic compounds and they are oxidized with O₂ which contains F₂ O.Alternatively, F₂ can be introduced in the oxidation stage, if desired.It is preferred to effect the oxidation by way of a reaction system inwhich hydrogen or H₂ O is not present such as a high frequency plasmasince HF is not thereby produced.

The rod F or pipe G shown in FIG. 4(a) is another embodiment of thisinvention in which an additional SiO₂ layer or water-repellent glasslayer is deposited further on the outer surface of the rod D or pipe Eshown in FIG. 1(a), wherein reference numbers 1, 2 and 3 represent thesame components as those shown in FIG. 1(a).

The layer 4 can be deposited in just the same manner as layer 2 byoxidizing SiCl₄ to SiO₂, or by applying glass frit having a similarcoefficient of expansion.

The rod F or pipe G in FIG. 4(a ) can also be made using another methodwhere a rod D or pipe E as shown in FIG. 1(a) is inserted in awater-repellent glass pipe or a fuse silica pipe 4 and then this pipecontaining rod D or pipe E is heated to a high temperature and pulled atboth ends to collapse the gaps between the rod D or pipe E and the pipe4. The rod F and rod G or pipe G can also be made in different ways. Forexample, the rod F in FIG. 4(a) can be made using a method in which adoped fused silica containing B₂ O₃ or F is deposited on the innersurface of a water-repellent glass pipe or a fused silica pipe 4, and apure fused silica rod having a clean surface or a pure fused silica rodon to which doped a fused silica containing B₂ O₃ or F has beendeposited is inserted in the deposited pipe and then this pipecontaining the rod is heated to a high temperature and pulled at bothends to collapse the gaps between the rod and pipe.

The pipe G in FIG. 4(a) can be made using a method where a doped fusedsilica containing B₂ O₃ or F is deposited and then a pure fused silicais deposited on the inner surface of the water-repellent glass pipe or afused silica pipe 4. The rod G in FIG. 4(a) can be made using a methodwhere a doped fused silica containing B₂ O₃ or F is deposited, a purefused silica is deposited and the doped fused silica containing B₂ O₃ orF is deposited alternatingly on the inner surface of the water-repellentglass pipe or fused silica pipe 4, and then this pipe or alternatively,this pipe into which a doped fused silica rod containing B₂ O₃ or F hasbeen inserted, is heated to a high temperature and pulled at both endsto collapse the gap between the pipe and the rod or cavity of the pipe.

The rod F and the pipe G in FIG. 4(a) are spun into fibers A and C shownin FIG. 4(b) when the cavity of pipe G is collapsed.

The pipe G in FIG. 4(a) is also spun into fiber B₁ shown in FIG. 4(b)when the cavity is not filled, while the rod G in FIG. 4(a) is spun intofiber B₂ in FIG. 4(b).

The invention will now be explained in greater detail by reference tothe following non-limiting examples thereof. Unless otherwise indicatedall parts, percents, etc. are by weight.

EXAMPLE

A process of this invention will be described by way of an experimentalexample. In the apparatus shown in FIG. 3, Ar gas, selected as a carriergas, was fed at a flow rate of 2 l/min, carrying BBr₃ and SiCl₄ to theburner while the temperature of the evaporator 5 was kept at 30° C. 60l/min of hydrogen gas and 45 l/min of oxygen gas were fed to the burnershown in FIG. 2. The outer surface of a pure fused silica rod of 10 mmin diameter was contacted with the burner flame and processed for 2hours to form a rod of about 20 mm diameter. The rod was heated in avacuum at 1300° C. for 2 hours and the rod was then spun by heating therod in a high frequency furnace to obtain a fiber having a core diameterof 75μ and a deposition layer diameter of 150 microns. On passing laserlight through this fiber, it was found that the light was completelytrapped with less scattering losses, and the entire transmission losseswere also low.

Charcteristics

The optical transmission fiber of this invention provides, as describedabove, a clad type fiber and an optical O guide of a highly pure fusedsilica portion in which the optical energy concentrates and asurrounding doped fused silica layer of a lower index of refractioncontaining B₂ O₃ or F and thus it possesses high optical transmissioncharacteristics and extremely low optical transmission losses.

Since completely oxidized SiO₂ or B₂ O₃ is deposited on the cleansurface of highly pure fused silica body in the doping with B₂ O₃ aswell as F, the interface is neither contaminated nor are gas bubblesformed (bubbles, if entrapped, can be eliminated by heating in vacuum orby application of supersonic waves under heating) thereby decreasing thescattering losses in the interface between the two fused silica mediahaving a different index of refraction.

In addition, the index of refraction can easily be controlled by varyingthe amount of B₂ O₃ contained in the fused silica. Moreover, the rawmaterials used in the process such as halides, hydrides or organiccompounds of B and Si as well as O₂ gas can be obtained in a highly purestate due to their physical and chemical characteristics thus reducingthe impurity content in the fused silica which contains the B₂ O₃. Thisdecreases the absorption losses and enables easily the preparation of afiber with a parabolic index of refraction distribution in which thetransmission losses are extremely low.

Since the inclusion of F does not substantially affect the lightabsorption, this process can provide a fiber having a transmission lossas low as that of the fused silica fiber, can provide an easy way tocontrol the index of refraction and can provide a transmission fiberhaving less overall transmission losses.

In one preferred embodiment, the transmission fiber according to thisinvention has a further SiO₂ layer deposited on the outer surfacethereof. In the SiO₂ layer containing B₂ O₃, its melting temperature islowered as the content of the B₂ O₃ is increased for reducing the indexof refraction, which decreases the viscosity of that portion resultingin a deformation in the shape thereof in the melting to spin. In orderto avoid this deformation and spin a fiber satisfactorily, an additionalSiO₂ layer is preferably deposited on this portion. A further effect isthat the index of refraction of the SiO₂ layer is lowered due to thetensile stress exerted thereon, after the spinning, because thecoefficient of expansion of the SiO₂ -- B₂ O₃ system is higher than thatof fused silica.

In the SiO₂ layer having F incorporated therein, the water-repellentglass layer is stable with respect to atmospheric conditions (primarilyfor humidity) at room temperature and inhibits water intrusion to theportion 2, which protects the doped fused silica doped with F portionfrom chemical attack by HF.

Further, the present process comprises a means to control the F contentin the SiO₂ and to control uniformly the dispersion of the F therein. Itcan also prevent the incorporation of hydrogen in the production stageand its effects on the melting to spin and thus the protection of thefiber from destruction can be obtained.

The optical transmission fiber according to this invention providesgreat advantages for communication cables used in optical transmission,connecting feeders between equipment, light guides, etc.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A tubular optical transmission fiber wherein therefractive index varies between the periphery and the centrallongitudinal axis of the fiber, said variation being symmetrical withrespect to the central longitudinal axis of said fiber, said fibercomprising an inner cavity, an outer radial portion being a lowerrefractive index portion being made of dope fused silica containing B₂O₃, said doped fused silica consisting essentially of SiO₂ and B₂ O₃ andan inner radial portion being a higher refractive index portion beingmade mainly of pure fused silica consisting essentially of SiO₂.
 2. Thetubular optical transmission fiber of claim 1, wherein a further fusedsilica or water-repellent glass is deposited on said fiber.
 3. Thetubular optical transmission fiber of claim 1, wherein the cavity withinsaid inner radial portion of said tube is filled with doped fusedsilica.
 4. The tubular optical transmission fiber of claim 3, wherein afurther fused silica or water-repellent glass is deposited on saidfiber.
 5. The tubular optical transmission fiber of claim 3, whereinsaid doped fused silica within said cavity of said tube contains B₂ O₃.6. The tubular optical transmission fiber of claim 5, wherein a furtherfused silica or water-repellent glass is deposited on said fiber.
 7. Anoptical transmission fiber wherein the refractive index varies betweenthe periphery and the central longitudinal axis of the fiber, saidvariation being symmetrical with respect to the central longitudinalaxis of said fiber, said fiber having at least one higher retractiveindex portion being made mainly of pure fused silica consistingessentially of SiO₂ and at least one lower refractive index portionbeing made of doped fused silica containing B₂ O₃, said doped fusedsilica consisting essentially of SiO₂ and B₂ O₃ and an outermost layerof a fused silica or a water-repellent glass layer.
 8. The opticaltransmission fiber of claim 7 wherein the innermost portion of saidfiber is pure fused silica.
 9. The optical transmission fiber of claim 7wherein the innermost portion of said fiber is pure fused silica and thecontent of the B₂ O₃ is radially changed in order to obtain a parabolicgradient of the radial distribution of the index of refraction.